Shoreline geomorphology and fringing vegetation of the Gippsland Lakes Paul Boon A, Neville Rosengren B, Doug Frood C, Alison Oates D & Jim E Reside AInstitute for Sustainability & Innovation Victoria University (Footscray Park campus) PO Box 14428, MCMC, Melbourne VIC 8001 BEnvironmental Geosurveys 1/22 Belmont Road, Ivanhoe VIC 3079 CPathways Bushland & Environment PO Box 360, Greensborough, VIC 3088 DOates Environmental Consulting, 2/44 School Avenue, Newhaven VIC 3925 EWildlife Unlimited PO Box 255, Bairnsdale, Victoria, 3875 Final − 28 October 2014 1 Contents 1. The Gippsland Lakes and its catchment − an introduction 3 1.1 Catchment geomorphology 4 1.2 Catchment geology 5 2. Geomorphology of the Gippsland Lakes 10 2.1 Origins 11 2.2 Sand barriers 15 2.3 Shoreline types 16 3. Fringing water-dependent vegetation of the Gippsland Lakes 20 3.1 Mangroves 22 3.2 Coastal saltmarsh 23 3.3 Other emergent woody vegetation 27 3.4 Other emergent non-woody vegetation 28 3.5 Fringing vegetation and freshwater subsides 31 4. The changing environment of the Gippsland Lakes 32 4.1 Early descriptions 32 4.2 The changing entrance to the lakes 35 4.3 Form and evolution 35 4.4 Salinity regimes & the permanent connection to the ocean at Lakes Entrance 36 4.5 River regulation and reductions in freshwater inflows 38 4.6 Sediment and nutrient loads 40 5 Environmental consequences of altered conditions 41 5.1 Shoreline erosion and retreat 42 5.2 Changes in fringing vegetation – loss of predominantly freshwater taxa 43 5.3 Changes in fringing vegetation – expansion of coastal saltmarsh and other halophytic taxa 47 6. Fringing vegetation and shoreline protection 48 6.1 The protective role of fringing vegetation 48 6.2 Re-instating fringing vegetation to protect coastal shorelines 49 7. The next steps 53 8. Cited references 53 2 1. The Gippsland Lakes and its catchment − an introduction The Gippsland Lakes consists of four large, shallow coastal lakes (Lake Wellington – 148 km 2; Lake Victoria – 78 km 2; Lake King – 97 km 2; and Lake Reeve – 52 km 2), fed by five major rivers: the Latrobe-Macalister-Thomson system, flowing into the western side of Lake Wellington; the Avon-Perry system flowing into the northern side of Lake Wellington; and the Mitchell, Nicholson and Tambo Rivers, all eastern rivers flowing into Lake King. Associated with the rivers and the shoreline of the four lagoons is a complex mosaic of fresh, brackish, and hyper- saline wetlands; the largest of these are the brackish-water Lake Coleman (~20 km 2), Dowd Morass (~15 km 2) and Macleod Morass (~5 km 2), and the ephemeral and often hyper-saline Lake Reeve wetlands. Of the remaining wetlands, only Sale Common (~2 km 2) remains naturally fresh. The combined surface area of the connected lakes and associated closed lagoons is 365 km 2 (Gippsland Coastal Board 2001). If the Gippsland Lakes is defined as extending from the western end of Lake Reeve to Red Bluff east of Lake Bunga, the catchment area contributing runoff to this system extends across ~20,300 km 2 of eastern Victoria (Figure 1), or ~10% of the total land area of the State. Figure 1: Digital elevation model (DEM) of Gippsland Lakes Catchment. Source: DEM from Department of Environment and Primary Industries, Victoria. 3 The Gippsland Lakes is a multi-component system with a wide range of marine, coastal, terrestrial, fluvial, and biological components and processes. Although the focus of the present study is the shorelines of the enclosed and semi-enclosed and linked lake basins that form the main body of the Gippsland Lakes, an understanding of the range of factors that contribute to the form and functioning of the lakes and its shorelines is needed to fully appreciate the geomorphological role played by fringing vegetation. Moreover, the past development of the lakes system and ongoing and future changes in the ecology and geomorphology of the lake shorelines are closely related to the evolution and dynamics of the Ninety Mile Beach and backing coastal sand barriers. Both the relict (inland) and active outermost (seaward) barriers determine the broad character of the system. They are very sensitive to changes in climate and sea-level on a short-term to long-term time scale. The broad character of the Gippsland Lakes and bordering lowland is determined by the topographical and geological character of the catchment, past and present climate, marine processes and changing sea-levels, the geomorphology and hydrology of inflowing rivers and the dynamics of contributing groundwater systems. Also relevant to understanding the present physical and associated biological systems that form the Gippsland Lakes are the direct, indirect, deliberate and inadvertent impacts of human activities. 1.1 Catchment geomorphology Descriptions of the landforms and geomorphological evolution of parts of the Gippsland Lakes catchment are included in several papers in McAndrew & Marsden (1971) and in the 1:250,000 geological report of VandenBerg & O’Shea (1981). Rosengren (1984) mapped the geomorphology and assessed geoscience sites across the entire catchment. The Victorian Geomorphology Review Group ( http://vro.depi.vic.gov.au/dpi/vro/ ) has prepared maps showing the geomorphology of Victoria at various scales including all the Gippsland Lakes catchment. Land system studies (geology, landforms, vegetation and soils) of the entire catchment include Nicholson (1978) and Aldrick et al. (1988). Also of relevance to the Gippsland Lakes are studies of the form and evolution of the upland areas of south-eastern Australia. The timing and causes of uplift and the development of the modern mountain and drainage systems determine the source and rate of transfer of sediment to the Gippsland Basin. A wide range of views has been proposed – ranging from the uplands as remnants of ancient higher mountains to being geologically young with several phases of uplift occurring over the Cainozoic, (Andrews 1933; Wellman 1979, 1987; Lambeck & Stephenson 1986; Galloway 1987; Ollier 1987; Holdgate et al. 2008; VandenBerg 2010). A number of studies of the catchment have concentrated on the dynamics of fluvial systems and the nature and causes of river channel changes (Erskine et al. 1990) and implications of changes in sediment supply and soil erosion on the Gippsland Lakes (Rutherfurd 1994; Wilkinson et al. 2005; Robinson & Sargant 2010). The Gippsland Lakes catchment displays a wide array of topography and slope morphology. The major streams that flow into the Gippsland Lakes originate in some of the highest terrain in south-eastern Australia, including areas that experience several months of snow cover. The catchment boundary between the headwaters of the Tambo River and Thomson River forms 4 part of the Great Divide in Victoria. The highest elevation in all the catchment is Mount Hotham (1862 m) at the headwaters of the Dargo River, a major tributary of the Mitchell River. Mount Bogong (1986 m) is north of the Divide and not part of the Gippsland Lakes catchment. Much of the northern catchment is over 1200 m elevation with extensive areas of tableland and elevated plains flanked by escarpments. Most of the upland terrain is a dense network of steep gradient streams and border ing valley slopes separated by narrow ridges. Locally deep weathering of rocks of varying resistance, slope movements (in part occasioned by periglacial freeze-thaw), and energetic fluvial action in response to tectonic uplift has combined to define the li thological and structural control of many landforms of the upland catchment. Long deeply incised valleys with gorge sectors and narrow floodplains are characteristic of all the major rivers and extend across the break -of-slope at the southern Palaeozoic bo undary across the plains of Neogene sediments that border the Gippsland Lakes. Between 100 and 150 m above sea -level is an abrupt boundary where t he elevated terrain descends to the plains and stepped terraces bordering the La Trobe River and the East Sale Plain and Munro Plain north of the Gippsland Lakes (Figure 2) . This boundary defines the southern limit of Palaeozoic rock outcrop ping across Gippsland (Figure 3). Figure 2: Profile drawn from DEM, Mount Hotham to Ocean Grange, showing elevation, tablelands, ridge & valley terrain and lowland plains . Source: DEM from Department of Environment and Primary Industries, Victoria. 1.2 Catchment geology There is an extensive lite rature dealing with aspects of the geology of Gippsland including the area of the Gippsland Lakes catchment. Several papers in McAndrew and Marsden (1971) and the comprehensive analysis of VandenBerg et al. (2000) and Birch (2003) provide stratigraphic and structural analysis of Palaeozoic basement rocks and reconstructions of the tectonic evolution of Gippsland. These regional analyses incorporate the earlier literature including the pioneering work of Howitt (1875, 1876). The Gippsland B asin has been a focus for detailed studies initially targeting the extent and environments of deposition of onshore brown coal, and since the 1950’s the offshore hydrocarbon potential (Hocking 1976). Detailed stratigraphy of the Seaspray Group and reconstr uction of ancestral river and barrier systems has been the subject of recent studies e.g. Holdgate et al . (2003) and Mitchell et al . (2007). 5 Figure 3: Geology of Gippsland Lakes catchment. Source: 1:500,000 Geology Eastern Victoria, Geoscience Victoria. Basement geology of the highlands The elevated areas of the northern catchment are developed on Palaeozoic sediments and metasediments of the Tasman Fold Belt System, intruded by granitic plutons of Silurian to Late Devonian age (Figure 3). The largest area of granitic terrain is the Baw Baw granites in the headwaters of the La Trobe and Thomson Rivers, with moderately sized bodies in the Mitchell, Nicholson and Tambo valleys. The basement geologies are dominated by siliciclastic sediments including thick sandstone and mudstone units.